1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic apparatus.
2. Related Background Art
Of the various systems employed for image forming apparatuses, including electrophotographic systems, thermal transfer systems and ink-jet systems, the electrophotographic systems, which, above all, supply higher speeds, better image quality and lower noise, are demonstratively superior to the others and are widely employed.
But among the electrophotographic systems, variations also exist, and include, for example: conventionally known multiple transferring systems and intermediate transferring member systems; multiple developing systems, for superimposing color images on the surfaces of photosensitive members and collectively transferring the images to form final images; and in-line systems, for arranging in series multiple image forming means (process stations) employing different colors and for transferring developed images to transferring materials using transfer belts.
Of these electrophotographic systems, the in-line systems are superior because they provide high processing speeds and because, since they require only a small number of transfer revolutions, there is less deterioration of the quality of the images they produce.
FIG. 20 is a diagram showing the partial configuration of a conventional image forming apparatus that uses an in-line system. In FIG. 20, an electrostatic adsorption transfer belt (hereinafter referred to as an ETB) 1 is fitted around a drive roller 7, an opposite adsorption roller 6 and tension rollers 8 and 9, and is rotated in the direction indicated by an arrow.
Process stations 201 (yellow), 202 (magenta), 203 (cyan) and 204 (black), which are processors that correspond to components of the invention, are arranged in series, and photosensitive members at the process stations 201 to 204 maintain contact with transferring rollers 3 via the ETB 1.
An adsorption roller 5, which is located upstream of the process stations, maintains contact with the opposite adsorption roller 6. A bias is applied to a transferring material that is passed through a nip formed by the adsorption roller 5 and the opposite adsorption roller 6, and while electrostatically attracted to the ETB 1, the transferring material is conveyed in the direction indicated by the aforementioned arrow.
The design of the conventional ETB 1 provides for a surface layer composed of urethane rubber, wherein a fluorocarbon resin such as PTFE, for example, is diffused, to be deposited on a base layer that is formed of a resin film, such as PVdF, ETFE, polyimido, PET or polycarbonate, having a thickness of 50 to 200 xcexcm and a volume resistivity of 109 to 1016 xcexa9cm, or on rubber, such as EPDM, having a thickness of 0.5 to 2 mm.
The image forming processing will now be described. First, the image forming process performed by a process station will be explained while referring to FIG. 21. FIG. 21 is a diagram showing the configuration of one of the process stations with which the image forming apparatus in FIG. 20 is equipped. The yellow process station 201 is employed in this explanation; however, the same process is performed by the other process stations.
In the configuration of the process station in FIG. 21, a photosensitive member 211, which is an image bearing member that corresponds to a component of the invention, is uniformly electrified by an electrifier 212, electrification means that corresponds to a component of the invention. A latent image is formed on the photosensitive member 211 by the emission of a scan light 214 by a light exposure system 213.
This latent image is developed by a developing roller 215, which corresponds to a developing device component of this invention, using toner contained in a toner container 216, and a toner image is formed on the photosensitive member 211. Residual toner, which is not transferred during a transferring process that will be described later, is scraped off by a cleaning blade 217 and is collected in a waste toner container 218.
The transferring process will now be described. When an OPC photosensitive member having a negative polarity is employed as the photosensitive member, the commonly employed inverted developing system uses negative toner to develop the light exposed portion. Therefore, a positive transfer bias is applied to the transferring roller 3 by a bias power source 4. In this case, a low resistant roller is generally employed as the transferring roller 3.
In the actual printing process, while taking into account the speed at which the ETB 1 moves and the distance between the transferring positions of the process stations, the image forming and transferring process for the process stations and the conveying of the transferring material are performed at a timing whereat the positions of the individual color toner images formed on the transferring material are matched, and after the transferring material has been passed through all the process stations 201 to 204, a toner image is formed on the transferring material. After the formation of the toner image on the transferring material is completed, the transferring material is passed through a well known fixing apparatus (not shown), and the toner image is fixed thereon.
After the above process has been completed, the ETB 1 is de-electrified by a charging/charge eliminating device 11, and is ready for the next print process.
The image density varies depending on the temperature and humidity conditions whereunder the image forming apparatus is used, and the usage condition of the process stations. In order to compensate for image density changes, control of image density is exercised. The image density control process will now be described.
To control image density, conventionally, means is employed for forming density patch images of individual colors on the photosensitive member, an intermediate transferring member (hereinafter referred to as an ITB) or the ETB, and for permitting a density sensor 13, which is a detection means component of the invention, to read the density patch images and feed them back to the process forming condition, such as a high voltage condition or the power for a laser, so that the maximum densities of the individual colors and the halftone characteristics match.
Generally, the density sensor 13 employs a light source to irradiate the density patch, and employs a light receiving sensor to detect the intensity of reflected light. A/D conversion is performed for the signal indicating the intensity of the reflected light, and the obtained signal is processed by a CPU 15, which constitutes means for controlling detection pattern forming means, which is a component of the invention, and the image forming condition, and the results are fed back to the process forming condition.
The image density control process has the objectives of constantly maintaining maximum individual color densities (hereinafter referred to as Dmax control) and of linearly maintaining the halftone characteristic of an image signal (hereinafter referred to as Dhalf control).
Dmax control can effectively maintain a constant balance between the individual colors, and can prevent dispersion or a color-superimposed character fixing failure due to the piling up of too much toner.
Specifically, in the Dmax control process, multiple density patches formed under different image forming conditions are detected by an optical sensor, a condition wherein a desired maximum density can be obtained is calculated by using the detection results, and the image forming condition is changed. In most cases, it is preferable that the density patch be formed using halftone.
For this reason, when a so-called solid image is detected, the width of a change in the sensor output relative to a change in the toner quantity is reduced, and satisfactory detection accuracy can not be obtained.
During the Dhalf control process, image processing is performed for canceling the non-linear input/output characteristic (xcex3 characteristic), unique to electrophotography, and for maintaining the linear input/output characteristic, to prevent a phenomenon that occurs due to the non-linear xcex3 characteristic, the output density is shifted to an input image signal and a natural image can not be formed.
Specifically, multiple density patches for which different input image signals are provided are detected by an optical sensor, and the relationship between the input image signal and the density is obtained. Then, an image signal input to the image forming apparatus is converted by the controller of the image forming apparatus, so that in accordance with the relationship a desired density is obtained from an image signal received from a host computer. Generally, Dhalf control is exercised after Dmax control has determined the image forming condition.
The process station employs a cleaning process to electrostatically collect the density patch formed on the ETB. During the cleaning process, a bias having a polarity opposite to that of the toner is applied to the photosensitive member, the toner is attracted to the photosensitive member by the transferring unit, and, as well as the residual transfer toner, is scraped off by the cleaning blade 14.
However, the following problem afflicting the conventional technique has arisen. In the above description, generally the density sensor irradiates the density patch using a light source, and the light receiving sensor detects the intensity of the reflected light. This system is roughly divided into two systems.
A system for detecting an irregular reflection element (or component) of reflected light
A system for detecting a regular reflection element (or component) of reflected light
A system for detecting an irregular reflection element will now be described in detail. An irregular reflection element is a reflection element sensed as a color, and the quantity of the reflected light is characterized by being increased in accordance with an increase of the amount of color material in a density patch, i.e., the amount of toner (FIG. 16). FIG. 16 is a graph showing the relationship between the irregular reflection light quantity and the toner quantity that is used both for the conventional image forming apparatus and the image forming apparatus of this invention.
Further, the reflected light is also characterized by being uniformly diffused in all directions from the density patch (FIG. 17). FIG. 17 is a conceptual diagram showing the relationship between the emitted light that strikes the toner and the irregular light reflection, and applies both to the conventional image forming apparatus and the image forming apparatus of this invention.
The density sensor for detecting an irregular reflection element is designed so that, as is shown in FIG. 18, in order to remove the affect of the regular reflection element, which will be described later, the irradiation angle xcex1 differs from the light receiving angle xcex2. FIG. 18 is a diagram showing an example structure for the density sensor for detecting the irregular reflection light.
However, when this density sensor for detecting irregular reflection is used to detect the density of black toner, no reflected light can be detected by the density sensor because black toner absorbs light.
Therefore, in this case, a method is proposed whereby a colored background, for example, is employed for the density patch, and the amount of light reflected from the background and hidden due to the black toner is measured, so that the density of the black toner can be detected.
When the in-line image forming system, including multiple photosensitive members, as explained previously for the prior art, is employed, as one example method, the formation and detection of a density patch on the photosensitive member are not performed in order to reduce the number of density sensors, a density patch formed on the ETB or the ITB, and only one density sensor is used to detect the densities of all the colors.
However, the resistance values of the transfer belt and the intermediate transfer member must be adjusted, so that the sheet feeding force and the image stability on the intermediate transferring member are maintained. Thus, in many cases, carbon black is diffused, depositing a black or dark gray covering on the transfer belt and the intermediate transferring member.
Therefore, to detect the density of black toner on the ETB (ITB), light is not reflected either from the density patch or the background (or substrate), so that the density sensor for detecting an irregular reflection can not detect the black toner. Thus, the density sensor for detecting regular reflection light, which will be described later, must be employed.
The system for detecting a regular reflection element of reflected light will now be described in detail. The sensor for detecting regular reflection light detects light reflected at irradiation angle a in a direction symmetrical to the normal line of the background face (ETB face), as is shown in FIG. 3. FIG. 3 is a conceptual diagram showing the relationship between irradiated light and regular reflection light that is applied both for the conventional image forming apparatus and the image forming apparatus of the invention.
The quantity of irregular reflection light depends on the reflectivity that is determined by a refractive index inherent to the material of the background (ETB), and the surface condition thereof, and is sensed as gloss. The maximum quantity of the light is reached when no toner is present on the background.
When the density patch is formed on the background, as is shown in FIG. 4, the portion of the background whereon toner is present is hidden, and there is no reflected light. Thus, as is shown in FIG. 5, which shows the relationship between the quantity of toner in the density patch and the reflected light quantity, the quantity of the reflected light is reduced as the quantity of the toner is increased. FIG. 4 is a conceptual diagram showing the relationship between the irradiated light and the regular reflection light for a case wherein toner is present on the ETB, and FIG. 5 is a graph showing the relationship between the toner quantity and the quantity of the regular reflection light and is applied both for the conventional image forming apparatus and the image forming apparatus of this invention.
Since the density sensor for detecting regular reflection light mainly detects the light reflected by the background, but not the light reflected by the toner, the density detection is available regardless of the color of the toner or the background, and the density sensor for detecting regular reflection light is more effective than the density sensor for detecting irregular reflection light.
Further, since generally the quantity of the regular reflection elements is greater than the quantity of the irregular reflection elements, and the density sensor for detecting regular reflection light is superior in its detection accuracy, it is also preferable that the density sensor for detecting regular reflection light be used to detect the density of the toner on the photosensitive member.
When the surface of the background is changed as it is used, the quantity of reflected light also varies. Thus, for the density sensor for detecting regular reflection light the performance of a correction can be effective; for example, the quantity of light reflected from the density patch can be standardized by using the quantity of light reflected from the background, and converting the obtained light quantity into density information.
However, a problem has arisen when the density sensor for detecting regular reflection light detects color toner. As was previously described, when the density patch of color toner is irradiated with light, the quantity of irregular reflection light increases as the quantity of toner increases, and the reflected light is uniformly diffused in all directions.
Therefore, the light detected by the density sensor is the sum of the regular reflection elements and the irregular reflection elements, as is shown in FIG. 6. FIG. 6 is a conceptual diagram showing irradiated light and reflected light when color toner is detected, and is are applied both for the conventional image forming apparatus and the image forming apparatus of the invention.
The relationship between toner quantity and reflected light quantity is, as is shown in FIG. 19, the sum of the regular reflection characteristic indicated by a fine solid line and the irregular reflection characteristic indicated by a broken line, i.e., represents the negative characteristic indicated by a thick solid line. Therefore, the linearity required for the density detection can not be obtained, and the density detection accuracy is not sufficient. It should be noted that FIG. 19 is a graph showing the relationship between toner quantity and reflected light quantity when the color toner is detected by the density sensor for detecting regular reflection light, and is applied both for the conventional image forming apparatus and the image forming apparatus of the invention.
To resolve this problem, proposed are a system disclosed in Japanese Patent Application Laid-open No. 5-249787 and a system disclosed in Japanese Unexamined Publication No. Hei 6-250480. According to the first system, a density sensor or a light receiving element for detecting irregular reflection and a density sensor or a light receiving element for detecting regular reflection detection are provided, so that color toner is detected using the irregular reflection element, and black toner is detected using the regular reflection element. According to the second system, a polarization plate is provided in front of a light emitting element and a light receiving element, and only the regular reflection element is fetched by using a difference between the polarized conditions of the irregular reflection element and the regular reflection element. However, either system increases the cost of the density sensor.
It is one objective of the invention to provide an image forming apparatus wherein even a density sensor having a simple structure of a regular reflection detection type can accurately detect the density of toner.